<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>边框三角形</title>
</head>

<body>
    <div class="tri"></div>
    <style>
        .tri {
            box-sizing: border-box;
            width: 10px;
            border-top: 5px solid red;
            border-right: 5px solid transparent;
            border-bottom: 5px solid transparent;
            border-left: 5px solid transparent;
            margin-left: 1000px;
            margin-right: 1000px;
            margin: auto;
        }
    </style>

</body>

</html>